This invention relates to an actuator and more particularly to a valve and a method to control the actuator/valve, especially for use downhole.
Downhole actuators and valves may be used for a number of different downhole operations but require a control mechanism in order to control the valve. Such control mechanisms during drilling or certain other operations, such as coring or fracturing, are difficult to provide. For example, during drilling a borehole, the drill bit is normally mounted on a drill stem or string, which is rotated from the surface. Controlling a valve on such a drill string poses obvious problems.
In certain situations, the borehole may require to be reamed, that is the diameter thereof increased. This may be achieved through simply using a larger drill bit, but this is inefficient at best since only a lower section of the borehole is required to be of the larger diameter. In other situations the upper section of the borehole may already be cased with a tubular string having a narrower diameter than that required below the casing and so it is not possible to use such a larger drill bit. Thus to drill or ream such boreholes, retractable blades are attached to the outside of the drill string. The blades are retracted and then sent down the narrower upper section of the borehole, activated by a valve to extend outwards, and then used to ream the lower section of the borehole.
The activation of such a reamer is often done by a drop ball valve. A ball is dropped onto a ball seat provided on the valve which blocks flow therebelow. The pressure above the ball/ball seat is increased and this pressure may be used to activate the reamer and cause its blades to pivot outwards.
However, the drop ball method may not always be possible since there may not be a clear passage to the reamer—other intervening equipment, such as a filter, can block this passage. Moreover, there is often no way to recover the ball after use and so retracting the reamer's blades is difficult. Furthermore, it is difficult to cycle such a system with drop ball methods and therefore the string has to be pulled out of hole to be reset which can be a very costly and time consuming process.